clientv3/integration: fix "TestMaintenanceSnapshotErrorInflight"
clientv3/integration: fix "TestMaintenanceSnapshotErrorInflight"

Errors from gRPC should be typed *status.statusError

=== RUN   TestMaintenanceSnapshotErrorInflight
WARNING: 2018/05/29 11:43:21 grpc: addrConn.resetTransport failed to create client transport: connection error: desc = "transport: Error while dialing dial unix localhost:55815860381826373260: connect: no such file or directory"; Reconnecting to {localhost:55815860381826373260 0  <nil>}
--- FAIL: TestMaintenanceSnapshotErrorInflight (2.42s)
	maintenance_test.go:192: expected context deadline exceeded, got rpc error: code = DeadlineExceeded desc = context deadline exceeded

Signed-off-by: Gyuho Lee <3454051ee3341eb69bdf6fca40899cdad4d86496@gmail.com>

glevand/coreos--etcd,etcd-io/etcd,bdarnell/etcd,cockroachdb/etcd,glevand/coreos--etcd,xiang90/etcd,vimalk78/etcd,ajityagaty/etcd,sebrandon1/etcd,mqliang/etcd,bdarnell/etcd,rnaveiras/etcd,coreos/etcd,li-ang/etcd,rnaveiras/etcd,mkumatag/etcd,mkumatag/etcd,etcd-io/etcd,joshix/etcd,gyuho/etcd,dannysauer/etcd,ajityagaty/etcd,sinsharat/etcd,etcd-io/etcd,joshix/etcd,mkumatag/etcd,gyuho/etcd,mqliang/etcd,rnaveiras/etcd,vimalk78/etcd,xiang90/etcd,dannysauer/etcd,sebrandon1/etcd,mqliang/etcd,li-ang/etcd,sinsharat/etcd,ajityagaty/etcd,cockroachdb/etcd,coreos/etcd